mmc: sdhci-pic32: Make pic32_sdhci_probe_platform() void

pic32_sdhci_probe_platform() always returned 0, so there's no reason for
a return value.  In addition, pic32_sdhci_probe() checked the return value
for possible error which is unnecessary.

Convert pic32_sdhci_probe_platform() to a void function and remove the
return value check. Fix the following Coccinelle warning:

./drivers/mmc/host/sdhci-pic32.c:127:5-8: Unneeded variable: "ret". Return "0" on line 137

Reported-by: Tosk Robot <tencent_os_robot@tencent.com>
Signed-off-by: Kaixu Xia <kaixuxia@tencent.com>
Link: https://lore.kernel.org/r/1604847648-13036-1-git-send-email-kaixuxia@tencent.com
Signed-off-by: Ulf Hansson <ulf.hansson@linaro.org>